1. Field of the Invention
The present invention relates to an image forming apparatus, and in particular it is preferable applied to an image forming apparatus, such as an electrophotographic apparatus in which an image bearing member is repetitively used and a cleaning device is equipped for bringing a cleaning member into contact with an image bearing member and collecting a residual material, such as residual transfer toner, from the surface of the image bearing member.
2. Related Background Art
Various types of electrophotographic copiers have been in practical use. These copiers form an electrostatic latent image on the surface of an image bearing member, such as a photoconductive photosensitive member; clean the surface of the image bearing after the development process and the transfer process have been completed; and thereafter perform the latent image forming process again.
A system for bringing a flexible conductive roller into contact with an image bearing member and applying a high voltage in order to maintain a uniform potential for the image bearing member, i.e., a contact charging system, is practically employed for various image forming apparatuses.
Compared with a charging system that uses Colutron, the contact charging system can suppress the occurrence of ozone and can effectively prevent smearing of an image.
Recently, however, to extend the service lives of the image bearing members used for many image forming apparatuses, such methods have been employed that reduce an amount of the surface scraping to which the image bearing members are subjected, e.g., the surface hardness of an image bearing member is increased, the primary charging at a high voltage is not applied in a time period other than the image forming period, or even when the primary charging is applied, it is applied at a lower voltage in that time period.
This is because the amount of the scraping of an image bearing member is greater when a high voltage is applied to the surface of the image bearing member by a primary charging device, as compared when it is not applied.
However, the above described contact charging system is a method used to superimpose a direct-current voltage on an alternating-current voltage, and upon the application of a high voltage, a nitrogen oxide such as NOx, called a discharge product, is generated and is attached to the surface of the image bearing member. Furthermore, the amount of the discharge product that is attached is increased as the time for the discharge period is increased.
It is well known that the sliding performance of the image bearing member is greatly deteriorated due to the attached discharge product. Therefore, generally, a method is employed whereby a cleaning blade or a sponge roller, as a cleaning member, is brought into contact with the image bearing member and scrapes off the discharge product attached to the surface of the image bearing member, as well as scraping the surface of the image bearing member that is deteriorated as a result of the application of a high voltage during the primary charging.
However, for a conventional image forming apparatus that employs an image bearing member having a rough surface or an image bearing member of which an amount of scraping is reduced, a balance between the amount of the attached discharge product and the scraping function performed by the friction member cannot be kept, and the discharge product will remain on the surface of the image bearing member.
When the printing is restarted while discharge product remains on the surface of the image bearing member, the sliding performance of the cleaning blade across the image bearing member is deteriorated, and since contact by the cleaning blade is not uniform, so-called “chattering” occurs.
When the blade “chattering” occurs, it is difficult to efficiently scrape off the material attached to the surface of the image bearing member, and accordingly, the attached material accumulates more easily on the surface of the image bearing member. As a result, the sliding performance of the drum is gradually deteriorated due to the durability of the attached material, and finally, the contact portion of the blade is inverted and a so-called “warped” phenomenon occurs.
The present inventor eagerly studied means for coping with these conventional problems. An overview of the study performed will now be described.
Through experiments and a study of the experiments, the present inventor found that the ratio of the applying time period for charging to a time period (hereinafter referred to as an idle rotation period) other than the former time period, such as the delivery of transfer materials to the outside of the apparatus after the transfer process has been completed, has a correlation with the sliding performance of an image bearing member.
According to the view of the present inventor, this phenomenon is caused by the fact that a material, such as a discharge product generated during image forming, attached to the surface of the image bearing member is scraped off by a blade during the idle rotation period, wherein a new discharge product is not generated.
Specifically, in a mode where a ratio of the idle rotation period is comparatively large, such as a case where the intermittent printing of a small number of sheets is conducted, the sliding performance of the image bearing member is maintained, so it is kept close to the initial state until the service life of the image bearing member has expired. On the other hand, during continuous printing, a high voltage is always applied during the image forming, so that the ratio of the idle rotation period to the charging period is reduced. Therefore, in accordance with the durability of the relevant material, the performance of the sliding image bearing member is deteriorated.
Therefore, the present inventor has continued to study the process eagerly, and resolved that, to overcome the deterioration of the sliding performance of the surface of the image bearing member, the material attached to the surface of the image bearing member must be frequently scraped off before a predetermined amount is reached, and that the sliding of the image bearing member should be maintained.
On the other hand, when the ratio of the idle rotation period to the applying time period for charging regarding a rotation time period of the image bearing member is greater than a predetermined ratio, e.g., when the number of times of printings for each sheet is relatively large among a predetermined number of time of printings, it can be assumed that an attached material, such as a discharge product, is appropriately scraped off from the surface of the image bearing member by a cleaning blade. Therefore, an idle rotation period is not required.